A variable capacitance element using an electrostatic actuator is an electronic part in which the MEMS (Micro Electro Mechanical System) technique is used.
A variable capacitance element using an electrostatic actuator includes, for example, a fixed electrode and a dielectric layer laminated over a substrate and a movable electrode arranged over a surface opposite the fixed electrode with an air layer therebetween. The movable electrode is elastically supported over the substrate with an anchor. When a potential difference arises between the fixed electrode and the movable electrode by an external power supply, electrostatic attraction causes a change in the distance between the fixed electrode and the movable electrode. As a result, electrostatic capacity changes.
With a digital variable capacitance element (having two capacitance values according to driving voltage), minimum capacity is obtained in a state in which a fixed electrode and a movable electrode are separate from each other (OFF state). On the other hand, maximum capacity is obtained in a state in which a fixed electrode and a movable electrode are touching with a dielectric therebetween (ON state).
Japanese Laid-open Patent Publication No. 2008-130613
Japanese Laid-open Patent Publication No. 2010-135614
With plural-bit variable capacitance circuits in which each bit includes the above digital variable capacitance element and a fixed capacitance element, capacitance values in a determined range are obtained with resolution corresponding to a bit number by adjusting the value of a fixed capacitance element of each bit. With plural-bit variable capacitance circuits, however, a capacitance value of a fixed capacitance element of a high order bit becomes larger than a capacitance value of a fixed capacitance element of a low order bit in order to obtain a greater change in capacity. If a fixed capacitance element of each bit includes a dielectric of the same thickness, then the electrode area of the fixed capacitance element of the high order bit becomes larger. As a result, circuit area increases.